This invention relates to a polishing method for a thin plate substrate (hereinafter referred to as a wafer) such as a liquid crystal substrate or a semiconductor substrate, a cleaning method for a wafer, and a protective film for a wafer.
Conventionally, there is a single side polishing method for a wafer wherein a wafer holding surface of one side surface of the wafer is held on a wafer holder, and a wafer polishing surface of the other side surface thereof is pushed onto a turn table under a pressure, while dripping a polishing agent, thereby polishing the wafer polishing surface. In this single side polishing method for a wafer, the polishing operation is performed without protection of the wafer holding surface. Further, there is known a cleaning method in which a wafer is cleaned with an alkaline solution to remove a polishing agent and others used in wafer polishing.
In the conventional single side polishing method, there is a problem that a wafer holding surface is etched or contaminated by a polishing agent because the wafer is polished without protection of the wafer holding surface.
With the foregoing problem of the prior art in view, it is an object of the present invention to provide a wafer polishing method in which, in single side polishing, covering a wafer holding surface (one side surface) of a wafer with a protective film, a wafer polishing surface (the other side surface) thereof is polished, so that the wafer holding surface can be prevented from being etched and contaminated by a polishing agent; a wafer cleaning method in which the protective film that remains behind on the wafer holding surface after polishing can be efficiently removed, and a waste cleaning solution is easily treated; and a wafer protective film that sufficiently covers the wafer holding surface in polishing but is effectively removed in cleaning.
In order to solve the above problem, a wafer polishing method of the invention in which a wafer holding surface of one side surface of a wafer is held and a wafer polishing surface of the other side surface thereof is polished while dripping a polishing agent, is characterized in that the wafer polishing surface is polished while covering the wafer holding surface with a synthetic resin protective film of etching resistance against the polishing agent, good adhesiveness to the wafer and a property easy to peel off from the wafer after polishing. As the synthetic resin protective film, polyvinyl butyral (PVB) resin is preferable.
PVB resin is known as a kind of polyvinyl acetal resin. A polymerization degree of PVB resin is preferably in the range of from about 300 to about 1000. Considering uniformity of the resin on the wafer surface and flatness of the wafer after polishing, a thickness of the protective film is preferably in the range of from 70 nm to 500 nm.
The above described PVB resin protective film is formed on the wafer holding surface under a procedure in which a solution containing PVB resin dissolved in isopropyl alcohol (IPA) is coated on the wafer holding surface, followed by heat treatment. As a coating method, spin coating is preferably employed. PVB resin and an IPA solution are preferably of high purity with a low content of impurities.
A PVB resin concentration in the IPA solution is preferably in the range of from 0.2 wt % to 5.0 wt %. The heat treatment is preferably performed under conditions of a temperature in the range of from 50xc2x0 C. to 200xc2x0 C. and a time in the range of from 30 seconds to 720 seconds. If the temperature is 50xc2x0 C. or lower, a protective film relatively easy to peel off is formed, while if the temperature is 200xc2x0 C. or higher, a protective film relatively hard to peel off is formed. Further, if a time of treatment is short, a film easy to peel off is readily formed, while if the time is long, the film hard to peel off is readily formed. Therefore, at a heat treatment temperature in the range of from 50xc2x0 C. to 200xc2x0 C., there can be formed a PVB resin protective film having a favorable performance hard to peel off during polishing but easy to be removed by cleaning.
The PVB resin has advantages that it is easily available because of its low price in comparison with other resin, and a highly flat wafer is obtainable because a film thickness is controllable by adjusting an amount of PVB resin dissolved in IPA. A PVB resin protective film is excellent in chemical resistance and surface hardness, and good in adhesiveness to glass and metal, so particularly suitable for a protective film.
A wafer cleaning method of the present invention is to clean a wafer already polished by the above described wafer polishing method, comprises: the first cleaning step of cleaning the polished wafer with ozone gas dissolved water; and the second cleaning step of cleaning the wafer already cleaned with the ozone gas dissolved water using an alkaline solution, wherein the polishing agent and the synthetic resin protective film are cleaned and removed from the polished wafer.
An ozone concentration in the ozone gas dissolved water is properly 10 ppm or higher and water into which fresh ozone gas is dissolved is constantly supplied.
Two ways are available for cleaning a wafer with the ozone gas dissolved water: one is to jet the cleaning water to the wafer, and the other is to immerse the wafer in the cleaning water.
In the alkaline cleaning, aqueous ammonia/hydrogen peroxide solution cleaning is preferably adopted, but NaOH/H2O2 and the like can also be employed. However, using ammonia water or NaOH only, a wafer surface becomes rough, so an alkaline solution containing hydrogen peroxide is employed.
Brush cleaning in combination with the alkaline cleaning can further increase a cleaning efficiency.
Using cleaning with ozone gas dissolved water only, it takes a long time to remove perfectly a synthetic resin protective film covering a wafer. However, the synthetic resin protective film can be easily removed by cleaning the wafer with ozone gas dissolved water for a short time (several minutes), followed by cleaning with an alkaline solution.
Cleaning with an alkaline solution is desirably cleaning with a heated aqueous ammonia/hydrogen peroxide solution (SC-1). After cleaning with the alkaline solution, the wafer is rinsed with pure water and dried, while the pure water rinsing followed by brush cleaning increases a power of removing the PVB protective film. The brush cleaning is especially effective for removal of the PVB protective film spreading onto the edge surface of the wafer.
For the brush cleaning, it is preferable to employ brush cleaning with a heated aqueous ammonia/hydrogen peroxide solution. Cleaning with an alkaline solution can dissolve and remove a synthetic resin protective film decomposed in cleaning with ozone gas dissolved water. For dissolution and removal of a synthetic resin protective film with an alkaline solution, cleaning with ozone gas dissolved water plays an important role wherein the higher an ozone concentration, the easier the dissolution of the synthetic resin protective film in the alkaline solution cleaning. Further, the longer a time consumed in cleaning with ozone dissolved wafer, the easier the dissolution and removal of the synthetic resin protective film. If cleaning with ozone gas dissolved water is not applied but only cleaning with an alkaline solution is applied, there arises a problem that the synthetic resin protective film is peeled off into filmy pieces which contaminate the cleaning solution or reattach onto the wafer.
It has been known that PVB can be removed with organic solvent or the like. The use of organic solvent, however, gives rise to trouble in waste liquid treatment, that is, an environmental aspect. In a wafer cleaning method of the present invention, ozone gas dissolved water and an alkaline solution are employed, making the waste liquid treatment easy.
A wafer treatment method of the present invention comprises: a step of polishing a wafer polishing surface of a wafer while holding a wafer holding surface thereof by the above described wafer polishing method; and a step of cleaning the polished wafer to remove a polishing agent and a synthetic resin protective film from the wafer by the above described wafer cleaning method.
The first wafer protective film of the present invention is formed on a wafer holding surface of a wafer with a synthetic resin of etching resistance against a polishing agent used in polishing the wafer, good adhesiveness to the wafer and a property easy to peel off from the wafer after polishing.
The second wafer protective film of the present invention is coated on the wafer holding surface in the above described wafer polishing method, and removed from the wafer holding surface by the above described wafer cleaning method.
Function
According to a wafer polishing method of the present invention, coating specific synthetic resin, preferably PVB resin, on a wafer holding surface, a wafer polishing surface is polished, so that the wafer holding surface can be protected from a polishing agent. In a wafer cleaning method of the present invention, a wafer, a holding surface of which is coated with synthetic resin, for example, a PVB protective film, and a polishing surface of which is polished, is cleaned with ozone gas dissolved water, so that the synthetic resin, for example, the PVB protective film, can be dissolved and removed using an alkaline solution.
Further, according to a wafer cleaning method of the present invention, an oxide film is formed on a surface of a wafer polished with ozone gas dissolved water, and a polishing agent and the like can be removed together with synthetic resin, for example, a PVB protective film with an alkaline solution. Removal of synthetic resin, for example, a PVB protective film with ozone gas dissolved water and an alkaline solution can be performed more easily than removal with aqueous sulfuric acid/hydrogen peroxide solution or organic solvent. Also, the treatment with ozone dissolved water and an alkaline solution is advantageously ecology-friendlier than treatment with a strong acid and organic solvent.